1. Field of the Invention
This invention relates generally to fuel ignition systems for valve controlled burners, and more particularly to direct ignition systems that use an electrical resistance igniter element in conjunction with a fuel valve and a thermal switch to prevent fuel flow to the burner until the igniter element reaches the fuel ignition temperature.
2. Description of the Prior Art
Fuel burner ignition devices generally comprise two categories, i.e., indirect and direct ignition systems. The indirect ignition systems usually involve a pilot flame which impinges on a thermocouple. As long as the flame continuously heats the thermocouple, current flows through the actuating coil for the fuel supply valve. If the thermocouple is properly positioned with respect to the pilot flame, the current flow will be sufficient to actuate the supply valve and permit fuel flow to the burner. The fuel is then ignited by the pilot flame. One inherent disadvantage of such a system is the criticalness in positioning the thermocouple with respect to the pilot flame. If the thermocouple is not positioned so that it will be properly heated by the pilot flame, the fuel supply valve will not operate. Another disadvantage of such systems is that the pilot flame can be diverted away from the thermocouple or completely extinguished by an air draft. When this occurs, the fuel supply valve becomes inoperable, preventing any fuel flow to the burner. And lastly, these ignition systems waste a significant amount of fuel since a pilot flame must be burning at all times.
Some direct ignition systems utilize electrical sparks to ignite the fuel flowing from the burner. Electrical time delay circuits are usually used in conjunction with these spark ignition systems to terminate fuel flow if the fuel is not ignited within a certain period of time. These systems also require a flame detection device to provide an indication to the timing circuit as to whether the fuel has been ignited. Such detection devices are costly to install and difficult to maintain.
Other direct ignition systems utilize glow plugs or igniter elements which are heated to the fuel ignition temperature and then used to ignite the fuel flowing from the burner. An inherent problem with these systems is that the electrical control circuits associated therewith are current responsive, and thus the fuel supply valve may be actuated prematurely if the igniter draws a larger current than desired before it reaches the ignition temperature of the fuel. Such a condition can be caused by design and manufacturing tolerance variations associated with the production of the igniter. Premature actuation creates a very unsafe condition since the fuel admitted to the burner will not be ignited and will be permitted to escape into the surrounding atmosphere.